Collisional parameters of H₂O lines: Effect of temperature

High-resolution Fourier transform spectra of H₂O -air mixtures have been measured at 296, 742, and 980 K for total pressures of 0.5 and 1.0 atm in the region of the ν₂ band. From these recordings, air-broadening coefficients have been determined for the doublet lines with K_c= J . From these data and previous experimental determinations, line-broadening parameters γ(T) are obtained at all three temperatures for the doublet lines from J = 0 up to 16. These values are then used to deduce the temperature-dependence exponent n such that γ(T) = γ(296) × (296/T)ⁿ. The values of n obtained using the two elevated temperatures are consistent and show that line widths decrease with T for small J-values (n ≈ + 0.8), are almost temperature independent on T for J ≈ 10 , and increase with T for high J lines (n ≈ - 0.3 for J ≈ 15). This original result, which confirms theoretical predictions made about 15 years ago with a semi-classical approach (J. Chem. Phys. 86 (1987) 144), is analyzed here using an improved version of the model. Comparisons between measured and computed widths show that calculations cannot correctly reproduce the experimental results at all temperatures simultaneously. This failure is attributed to the use of an inappropriate interaction potential and to the very strong dependence of the broadening γ(v) on the relative velocity v for high J lines. This strong variation of γ(v) raises the question (which is discussed here) of what calculated quantity is to be compared with measured values (i.e. γ [v̄, [γ(v)] (T) , or some other more complex mean to account for the velocity averaging of the spectral profile itself?). Nevertheless, when the temperature exponent n is considered, satisfactory agreement between experiments and predictions is obtained, particularly when the averaging of widths over the relative velocity distribution is made.